1. Field of the Invention
This invention relates generally to a method and compositions for forming thin film ferrite layers on substrates. More particularly, the invention relates to a method and compositions for forming thin film ferrite layers with metalorganic chemical vapor deposition (MOCVD) using a liquid delivery technique for flash vaporization of the precursor chemistry.
2. Description of the Related Art
High quality ferrite films possess numerous properties of technological importance, including high saturation magnetization, and magneto-optic effects. Two important ferrite families are the spinels, e.g., Ni--Zn ferrite, (Ni,Zn)Fe.sub.2 O.sub.4, and Ni--Mn ferrite, (Ni,Mn)Fe.sub.2 O.sub.4 and the hexagonal ferrites, e.g. BaFe.sub.9 O.sub.11. The nickel-zinc and nickel-manganese materials are solid solutions in which the proportions of each metal can range very widely. The general formula for spinel type ferrite materials is MFe.sub.2 O.sub.4 where M=Mn, Fe, Ni, Co, Cu, Mg, and Zn. The ferrites can also be mixed to optimize their properties. Typical examples are the addition of ZnFe.sub.2 O.sub.4 to NiFe.sub.2 O.sub.4, the addition of ZnFe.sub.2 O.sub.4 to MnFe.sub.2 O.sub.4, and the addition of ZnFe.sub.2 O.sub.4 to CoFe.sub.2 O.sub.4. (Ni,Zn)Fe.sub.2 O.sub.4 is useful for high frequency applications, and (Mn,Zn)Fe.sub.2 O.sub.4 is useful as a soft magnetic material.
Other ferrites include the hexaferrites, so called because of their hexagonal crystal structure. These ferrite materials include BaFe.sub.12 O.sub.19 and SrFe.sub.12 O.sub.19.
In each of the ferrite systems, precise and repeatable compositional control is required in order to produce films of high quality. Physical deposition methods (e.g., sputtering, evaporation) to fabricate thin film deposition are deficient in this regard, as are traditional approaches to metalorganic chemical vapor deposition (MOCVD) involving the use of bubblers.
A great need exists to miniaturize discrete magnetic components. At present, discrete filters, limiters, phase shifters, isolators and circulators are large and costly when compared to the active circuits with which they are associated. Thin film-based devices are an attractive solution to this problem, but require cost-effective processing of films in the thickness range of from about 3 to about 30 microns, as well as low process temperatures (approximately 450.degree. C.) to carry out direct integration (for devices other than hybrid devices, where process temperature may not be a major issue).
A number of processes have been investigated to date for depositing ferrite films, including pulsed laser ablation deposition (PLD), sputter deposition, and spin spray processing (SSP). While high quality films have been produced in some cases, no technique has yet been demonstrated which has clear potential for the manufacture of commercially viable devices.
The high rate deposition techniques which have been used successfully for multicomponent oxide thin films are pulsed laser ablation deposition (PLD) and chemical vapor deposition (CVD). The use of PLD for ferrite films (Mn,Zn ferrite) has been investigated and the quality of films produced was acceptable. However, as the deposition area is scaled in PLD, one loses the deposition rate advantage that is inherent to PLD. Area scaling is important in making ferrite layers because, although device and chip dimensions are small, cost-effective deposition in an integrated process is ideally accomplished at wafer scale dimensions.
Sputter deposition has also been investigated for ferrite films for many years. However, sputter deposition has the drawback of low rates for oxide deposition, relatively low oxygen partial pressures during processing, and stoichiometry control issues related to preferential sputtering.
An innovative technique, termed spin spray processing (SSP), has been investigated by various researchers. This technique has the advantage of processing at 100.degree. C., but unfortunately the resultant film properties have not been as good as desired and film losses have been high in the practice of such technique.
It would therefore be a significant advance in the art, and is accordingly an object of the present invention, to provide a method for the formation of thin film ferrite materials by MOCVD which affords a close stoichiometric control of the product ferrite film, and which is able to be usefully carried out at low temperature process conditions.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure a nd appended claims.